Well, the STP site describes it as 600 fill power down. It is a light weight down jacket -- would need layering in the coldest weather if you were going to be outside for extended periods of time.
600 fill power. The jacket weight is 13.8 ounces. Yet, the jacket was warm enough today at zero degrees F. and wind to make it feel like -11 deg. F. Worn over a lightweight cotton flannel shirt.